When you press the shutter button halfway on a digital camera, both exposure and focus are locked simultaneously. This is usually, but not always, a good thing. We’ve all had those photos where the foreground in a scene looks great and the sky is totally washed out!
Metering with AE lock
Sometimes it’s preferable to lock exposure independently from focus. This can be accomplished using auto exposure lock (not all digital cameras have this useful feature).
Using AE lock on the XSI/450D
On the XSi/450D, focus first. Next, aim the camera where you want to lock exposure and press the (*) button on the top, back of the camera. Exposure is locked. Then, recompose and take the picture. [page 88 in the manual]. If you’re doing a close-up shot, watch the depth-of-field because recomposing could have an undesirable effect on the subject.
Focusing with AF lock a bit trickier
When the (*) is set to AE lock, the camera still locks focus and exposure together when half-pressing the shutter. However, this is not the case when the (*) is set to AF lock, where the camera locks focus when that button is pressed.
Because I take a lot of wildlife photos, I decided to switch the (*) button to AF lock [camera manual page #158: C.Fn-10 shutter/AE lock button]. I didn’t realize the camera no longer focused when half-pressing the shutter button.
No wonder my photos weren’t properly focused.
I should have known though, because there was no audible focus lock sound and no focus confirmation light. What did show in the viewfinder and on the LCD was a *. I now realize it was a reminder to hit the (*) button to focus.
Always something new to learn in digital photography.